In general, it is understood that a Fourier transform mass spectrometer (“FT-MS”) is an ion cyclotron resonance mass spectrometer (“ICR-MS”) where ion packets are excited to mass-specific cyclotron motions in a strong magnetic field, and the excited ions generate image currents in detection electrodes. The image currents are recorded as time signals (“transients”) and converted into a frequency spectrum by a Fourier transformation. The frequency spectrum may be converted into a mass spectrum since the cyclotron frequency is inversely proportional to the mass of an ion. The ions are trapped, radially by a magnetic field and axially by electric potentials, in an ion cyclotron resonance (“ICR”) measuring cell.
The magnetic field of an ICR mass spectrometer is typically generated by superconducting solenoids at liquid helium temperatures, and reaches field strengths of up to 15 tesla. As a result, ICR mass spectrometers have the best mass resolution and mass accuracy of all mass spectrometers since the magnetic field of a superconducting solenoid is stable, and frequency measurement is one of the most accurate prior art measurement methods. The cyclotron frequency may be shifted by space charge in the ICR measuring cell, which is generated by the ions. Simulations show that ion packets orbiting on cyclotron trajectories influence one another and, therefore, change shape in the course of the measurement as a result of interactions within individual ion packets and between different ion packets. The space charge, and thus the cyclotron frequencies of the ion packets, may be subject to a temporal drift during the measuring time. The electric potentials for axial trapping of the ions in the measuring cell also influence the cyclotron frequency and must be constant, at least during the measuring time. All types of parameter drifts during the measuring time lead to temporal frequency modulations in the ion current signal. This temporal frequency modulation causes the line widths in the frequency spectrum to increase (i.e., “smearing” the line), reducing the mass resolution. As a result, the smeared line may cause inaccurate mass determinations.
There are other classes of mass spectrometers where ion packets are stored in one spatial direction in a harmonic parabolic potential, and in the direction perpendicular to the harmonic parabolic potential by radial forces. The radial forces may be, for example, magnetic fields, pseudopotentials generated by RF fields, or electrostatic fields between central electrodes and outer shell electrodes. These types of mass spectrometers detect an oscillatory motion in the harmonic potential, in contrast to ICR mass spectrometers which detect the cyclotron motion. If the harmonic potential is spatially homogenous at right angles to the oscillatory motion, an ion packet containing ions of the same mass will keep its shape. Ions of different masses oscillate as coherent ion packets at different frequencies and induce image currents in detection electrodes. The image currents are detected with high time resolution. In ICR mass spectrometers, the recorded time signal is converted into a frequency spectrum using a Fourier transformation and changed into a frequency mass spectrum by a corresponding conversion of the frequency axis.
These classes of “oscillation mass spectrometers” includes the following embodiments:                three-dimensional RF quadrupole ion traps with detection electrodes for image currents as disclosed in U.S. Pat. No. 5,625,186 to Frankevich et al. and U.S. Pat. No. 5,283,436 to Wang;        linear RF quadrupole ion traps with detection electrodes for image currents, where the ions oscillate between two pole rods, and the detection electrodes are located between the pole rods, as disclosed in U.S. Pat. No. 6,403,955 to Senko),        an electrostatic ion trap, marketed by Thermo-Fischer Scientific (Bremen) under the name of “Orbitrap® electrostatic ion trap”, where the ions orbit in a radial electric field, on the one hand, and oscillate in a parabolic electric potential in a direction perpendicular to this, on the other hand. The necessary electric potentials are generated by an internal spindle-shaped electrode, which is held at an attractive potential, and an outer shell, to which a repulsive potential is applied.        
The ICR mass spectrometers and the oscillation mass spectrometers hereinafter will be referred to jointly as “frequency mass spectrometers” since, in both types, the motion of ion packets detected is temporally resolved (e.g., by image currents) and the recorded time signal is transformed into a frequency spectrum. The time signal is a superposition of different frequency components (i.e., time signals with different frequencies which are separated in the frequency spectrum) when ions of different masses are present.
The mass resolution of a frequency mass spectrometer increases—at least in theory—in proportion to the measuring time. In the Orbitrap® spectrometers and other commercially available ICR mass spectrometers, the measuring time for a time signal is typically between one tenth ( 1/10) of a second and a few seconds. These measuring times produce a high mass resolution in the order of R=m/Δm=100,000 for a given mass m=200 Dalton, where “m” is the mass and “Δm” is the full width at half-maximum (“FWHM”) of a mass signal. Typically, the mass resolution decreases with increasing ion mass for all frequency mass spectrometers, although in different proportions.
Frequency mass spectrometers generally require a strong enough vacuum such that the ion packets do not spread out by diffusion during the measuring time as a result of undergoing a large number of collisions. Furthermore, the instrument parameters of frequency mass spectrometers, such as the electric potentials at the electrodes or currents generating magnetic fields, and also internal parameters, such as the space charge or electrostatic charges on electrodes, must be as constant as possible during the measuring time to avoid frequency shifts. Any temporal parameter drift may cause broadening and shifting of the peaks in the frequency spectrum, which limits the mass resolution or the mass accuracy of the mass spectrum. One consequence of the relatively long measuring times is that it is difficult to keep all instrument parameters sufficiently constant. Furthermore, it may only be possible to influence internal parameters to a limited extent, if at all (e.g., for a space charge which changes over time as a result of interactions within ion packets or between ion packets).